The invention relates to a method for determining the humidity and/or density of a dielectric material in a resonator filled with the material, comprising a sender and a receiver.
The dielectric properties of a material, described by the complex relative dielectric constant ∈r=∈′r−i∈″r, can be affected by humidity and density in case of porous materials. Humidity and density thus modify the scalar parameters resonant frequency fr and resonator quality Q of a material-filled resonator in contrast to those of the empty, air-filled resonator (fr0 and Q0) in such a way that
            f              r        m              =                                        f                          r              0                                                          ɛ              r                                      ⁢                                  ⁢        and        ⁢                                  ⁢                  1                      Q            m                              =                        1                      Q            0                          +                              ɛ            r            ″                                ɛ            r            ′                                ,wherein 1≦∈′r≦∈′rmax and 0≦∈″r≦∈″rmax, wherein ∈′rmax and ∈″rmax are the maximum values resulting for the respective material from the assigned humidity range and density range.
In prior art, different methods are known for measuring the humidity or density of granular materials, in which the resonance behavior of a matter-filled resonator is used.
For example, from U.S. Pat. No. 5,666,061 a method is known for measuring the humidity in granular materials by means of microwaves. There, one electronically reacts to a threshold while bidirectionally sweeping a frequency range. From the chronological properties of pulses created in such a way the components of the dielectric constant are deduced.
All methods are relatively slow and have either a dependency of the density of the material or have relatively large errors in the determination of the humidity.
The invention is underlied by the problem to specify a method of the type initially mentioned, by which a fast, accurate and density-independent determination of the humidity is possible.